1. Field of the Invention
The present invention relates to a control apparatus and a system board comprising modules (or devices) controlled by the control apparatus, and more particularly, a system board and an impedance control method of the board for preventing the distortion of signals transmitted between modules (or devices) controlled by the control apparatus.
2. Description of the Related Art
In general, a system such as a computer comprises a control apparatus or controller and modules controlled by the controller. A typical example is a memory subsystem comprising memory modules and a memory controller. But, as data processing speed of a system increases and the amount of data to process grows, the number of modules to be plugged into a system board increases.
A system such as a computer comprises a mother board, where modules are plugged into connectors installed in the mother board. The number of modules plugged into the mother board increases according to the improvement of system performance, and an impedance of a signal line which the mother board itself has varies as modules are plugged into the mother board. This is because a unit capacitance of a transmission line varies by parasitic capacitance of the module.
In general, the distances between connectors installed in a mother board of a computer system are equal, and modules installed in these connectors are controlled by one control apparatus. Therefore, in the conventional system board, there is a great difference between a characteristic impedance of a signal line from a control apparatus to a first connector and a characteristic impedance of a signal line connected to other connectors in case those modules are plugged into the connectors. That is, the characteristic impedance of the signal line from the control apparatus to the first connector is larger than the characteristic impedance of the signal line connected to other connectors in case those modules are plugged into the connectors.
In this case, a signal distortion phenomenon can occur by a reflection due to the impedance difference in the process of signal transmission through the signal line.
A conventional system board will be described as follows with reference to the accompanying drawings. FIG. 1 is a block diagram showing a configuration of a mother board of a conventional computer system, which comprises a control apparatus 10, and n connectors CON1, CON2, CON3, . . . CONn. The n connectors CON1, CON2, CON3, . . . and CONn shown in FIG. 1 are installed in a mother board in order to plug n modules (not shown), and the control apparatus is installed in the mother board directly.
In FIG. 1, a signal line to transmit data DQ and strobe signal is arranged between the control apparatus 10 and the n connectors CON1, CON2, CON3, . . . and CONn. The length of a signal line from the control apparatus 10 to a first connector CON1 is l0, and the length of a signal line between connectors CON1, CON2, CON3, . . . and CONn is l0.
A mother board shown in FIG. 1 inputs and outputs data between the control apparatus 10 and a corresponding memory module plugged into the n connectors CON1, CON2, CON3, . . . and CONn in response to a strobe signal being output from the control apparatus 10.
Unlike the configuration shown in FIG. 1, the control apparatus 10 can be plugged into the mother board through a connector like modules, not being plugged into the mother board directly, and also, modules can be plugged into the mother board directly, not through n connectors CON1, CON2, CON3, . . . and CONn. That is, an impedance control method of a system board of the present invention to be described below is applied not only in the case of the embodiment shown in the above-mentioned FIG. 1
FIG. 2 illustrates a configuration modeling the block diagram shown in FIG. 1, which models the configuration where n modules M1, M2, M3, . . . and Mn are plugged into n connectors CON1, CON2, CON3, . . . and CONn, and k memory devices D1, . . . and Dk are loaded in each of n modules M1, M2, M3, . . . and Mn.
In FIG. 2, a serially-connected package P1 and an input driver IB1 and an output driver OB1 illustrate a control apparatus 10. A serially-connected stub resistor Rs and k memory devices D1, . . . and Dk illustrate each of modules M1, M2, M3, . . . and Mn, and serially-connected packages P21, . . . and P2k and input/output drivers IB21, OB21, . . . and IB2k, OB2k illustrate each of k memory devices D1, . . . and Dk. l0 and l0 indicate the length of signal lines on the mother board; l0 indicates the length of a signal line from a resistor Rm to a first connector CON1, and l1 indicates the length of a signal line from the first connector CON1 to a nth connector CONn.
RT1 and VT1 indicate a terminal resistor and a terminal voltage, respectively, to terminate a signal transmitted from one of modules M1, M2, M3, . . . and Mn to the control apparatus 10, and RT2 and VT2 indicate a terminal resistor and a terminal voltage, respectively, to terminate a signal transmitted from the control apparatus to the modules M1, M2, M3, . . . and Mn, respectively. That is, a signal transmitted from one of the modules M1, M2, M3, . . . and Mn to the control apparatus 10 is terminated by the terminal resistor RT1 and the terminal voltage VT1 having a specific voltage difference, the signal transmitted from the control apparatus 10 to the modules M1, M2, M3, . . . and Mn is terminated by the terminal resistor RT2 and the terminal voltage VT2 having a specific voltage difference.
The drawings of FIG. 1 and FIG. 2 are disclosed in 1998 Symposium on VLSI Circuits Digest of Technical Papers with a title of xe2x80x9c5G Byte/s Data Transfer Scheme with Bit-to-Bit Skew Control for Synchronous DRAMxe2x80x9d.
Looking at the length of a signal line of a mother board of the conventional computer system as shown in FIG. 1 and FIG. 2, the length of signal lines between n connectors CON1, CON2, CON3, . . . and CONn are equal, except the length of a signal line from the control apparatus 10 to the first connector CON1. Then, the variation of a characteristic impedance of a signal line in case that the signal is transmitted from the control apparatus 10 to the modules M1, M2, M3, . . . and Mn is as follows, by calculating the characteristic impedance of the signal line in case that the modules M1, M2, M3, . . . and Mn are not plugged into the mother board of the computer system and the characteristic impedance of the signal line in case that the modules M1, M2, M3, . . . and Mn are plugged into the mother board as shown in FIG. 1.
The characteristic impedance Z in the case in which the modules M1, M2, M3, . . . and Mn are not plugged into the connectors CON1, CON2, CON3, . . . and CONn of the mother board of the computer system as shown in FIG. 1, can be described as the following equation (1), if an inductance per unit length of the signal line of the mother board are L and C, respectively.                     Z        =                              L            C                                              (        1        )            
On the other hand, a characteristic impedance Zxe2x80x2 of the signal line in case that the modules M1, M2, M3, . . . and Mn are plugged into the connectors CON1, CON2, CON3, . . . and CONn of the mother board can be described as the following equation (2), if a parasitic capacitance of each of the connectors CON1, CON2, CON3, . . . and CONn is Cc and the distance between connectors is l1.                               Z          xe2x80x2                =                              L                          C              +                              Cc                11                                                                        (        2        )            
As known from equations (1) and (2), because the modules M1, M2, M3, . . . and Mn are plugged into the connectors CON1, CON2, CON3, . . . and CONn, the magnitude of a denominator of the characteristic impedance Zxe2x80x2 becomes larger than the magnitude of a denominator of the characteristic impedance Z of the signal line, and thus the magnitude of the characteristic impedance Zxe2x80x2 decreases. As a result, the characteristic impedance Z of the signal line from the control apparatus 10 to the first connector CON1 decreases to the characteristic impedance Zxe2x80x2 of the signal line after the first connector CON1.
FIG. 3 is a graph showing the variation of a characteristic impedance approximately of a signal line according to the distance of a mother board of a computer system shown in FIG. 1, wherein a characteristic impedance of a signal line to the first connector CON1 maintains Z and a characteristic impedance of a signal line after the first connector CON1 maintains Zxe2x80x2. That is, the characteristic impedance Zxe2x80x2 of the signal line after the first connector CON1 maintains an equal value because the length of signal lines between connectors CON1, CON2, CON3, . . . and CONn are equal.
A signal distortion phenomenon occurs in the process of signal transmission through a signal line because the characteristic impedance Z of the signal line of the mother board of the computer system as shown in FIG. 3 is reduced rapidly to the characteristic impedance Zxe2x80x2 after the first connector CON1. The signal distortion phenomenon worsens in the process of rapid variation of the characteristic impedance, that is, in the process that a signal is transmitted from the control apparatus 10 to the first connector CON1.
It is an object of the present invention to provide a system board that can prevent the signal distortion phenomenon by reducing the variation of characteristic impedance of signal lines between modules generated plugged into the system board.
It is another object of the present invention to provide an impedance control method of a system board in order to accomplish the above object.
According to one aspect of the present invention, there is provided a system board which comprises a plurality of modules and a controller for controlling the plurality of modules. Signal lines are connected from said controller to said plurality of modules and are arranged so that the length of the signal lines between the controller and the plurality of modules becomes shorter as a distance between each of said modules and the controller increases.
In one embodiment, the characteristic impedance of signal lines between the controller and the modules decreases exponentially as the distance between each of said modules and the controller increases. Each of said plurality of modules can be a module comprising a plurality of memory devices. Each of said modules can be a memory device. In one embodiment, a dummy capacitor is included in signal lines between said controller and said plurality of modules.
According to another aspect of the present invention there is provided a system board which comprises a plurality of modules and a controller for controlling the plurality of modules. Signal lines are connected from the controller to the plurality of modules, and each of the signal lines between the controller and said plurality of modules comprises a dummy capacitor such that characteristic impedance of the signal lines decreases as a distance between each of said modules and the controller increases.
In one embodiment, the characteristic impedance of the signal lines between the controller and the modules decreases exponentially as the distance between each of the modules and the controller increases.
Each of the modules can be a module comprising a plurality of memory devices. Each of said modules can be a memory device. In one embodiment, the magnitude of the dummy capacitor increases as the distance from the controller increases when the lengths of signal lines between the modules are equal.
According to another aspect of the present invention, there is provided an impedance control method for a system board. A plurality of modules are provided, and the modules are controlled. Signal lines are connected from the controller to the plurality of modules, and the characteristic impedance of signal lines between the controller and the plurality of modules decreases exponentially as the distance between each of the modules and the controller increases.
Therefore, the system board and the impedance control method of the board of the present invention can reduce the signal distortion phenomenon by configuring the signal lines such that the characteristic impedance of signal lines between the control apparatus and modules (or devices) decreases exponentially in case that modules (or devices) are plugged on the board.